An image of a scene captured with a camera usually exhibits some effects due to a depth-of-field of the optical system where content in the scene is captured with varying sharpness (i.e., focus) depending on the distance from the camera. The desirability of scene content varying in sharpness within an image depends on the type of image. For example, portraits where the background is blurred relative to the subject are typically preferred over portrait images where the background is as sharp as the subject. Conversely, landscape images with the entire scene sharp are usually preferred over landscape images with substantial sharpness variation.
The depth-of-field of the captured image varies with the size of the aperture in the taking lens. This is illustrated in FIGS. 3A and 3B, which show images of the same scene captured with two different aperture settings. FIG. 3A, illustrates a low F/# digital image 190 that was captured using a low F/# (large aperture), providing limited depth of field and blurring the background. FIG. 3B illustrates a high F/# digital image 195 captured using a higher F/# (smaller aperture), increasing the depth of field and producing a background in relatively good focus. (In this example, the low F/# digital image 190 was captured using an F/8 aperture setting and the high F/# digital image 195 was captured using an F/32 aperture setting.)
To control the depth-of-field for different scene types, the aperture setting to the lens can be controlled, either manually or automatically. FIG. 4 illustrates an example of an image capture process 200 that can be used to control the depth-of-field in portraiture applications. In this case, an identify portrait scene step 210 is used to determine that the image being captured is a portrait image. In some cases, this is accomplished by the photographer using appropriate controls on the camera to select a Portrait photography mode. In other cases, this can be done based on analysis of a preview image. When it is determined that a portrait image is being captured, a select maximum aperture step 220 is used to set the optical system to the maximum possible aperture setting (smallest F/#). This provides the minimum depth-of-focus, and therefore provides the maximum amount of blur for the background. Finally a capture archival image step 230 is used to capture and store an image of the scene.
U.S. Patent Application Publication 2003/0007076, to Okisu et al., entitled “Image-processing apparatus and image-quality control method” teaches ways to automatically select one of several scenes modes in a digital camera and adjust image capture settings for capture of an archival image. For a portrait mode an aperture-preferred exposure program and a relatively large aperture is selected. For a landscape mode an aperture-preferred exposure program is also selected, this time with a relatively small aperture.
U.S. Pat. No. 7,990,429 to Saito, entitled “Imaging Device with Blur Enhancement” addresses the problem of enhancing the blur in a captured digital image. Saito teaches segmentation of the image into a first region (subject) and a second region (background), followed by increasing the blur in the background region. In at least one embodiment, the amount of blur applied to the background region is a function of the original blur level in the background region such that the amount of additional blur applied during enhancement is increased for images having larger original blur levels. One limitation of this approach is that background blur is always increased. This does not acknowledge the possibility of an optimum level of background blur. A second limitation is the lack of any tie between estimated sharpness or blur and perceptual quality. While Saito teaches processes related to increasing the blur in an image, he does not teach a relationship between sharpness or blur and perceptual quality.
To capture a scene with a given field-of-view, the focal length of the camera taking lens scales with the size of the capture medium (film in the case of a film camera, or the image sensor in the case of a digital camera). The use of lenses with shorter focal lengths with smaller sensors enables use of smaller apertures for a given photographic condition. Thus, cameras with smaller sensors tend to capture scenes with larger depth-of-field than cameras with larger sensors. This is significant, since digital camera sensor size varies widely, yet people often prefer to have images that have the field-of-view and depth-of-field characteristics typically associated with a moderately large sensor.
There remains a need for a method to control the effective depth-of-field in a captured digital image that is independent of the limitations of the optical system, and to automatically adjust the depth of field characteristics in accordance with scene type.